1. Field of the Invention
This invention generally relates to tape recording/reproduction systems of the type having the capability of automatically finding a feature recorded on the magnetic tape by detecting the pauses between the recorded features, and thereupon of precisely positioning the selected feature relative to the tape head for replay. More particularly, this invention relates to an improved apparatus for, and method of, reliably distinguishing between interfeature pauses between features and intra-feature pauses during features in order to accurately control the selective replay of the recorded features.
2. Description of the Prior Art
It is customary to record many features or programs on a recording medium such as a magnetic recording tape. Automatic feature finder systems have been proposed to replay on tape recording/reproduction systems features which have been pre-selected by the listener, while bypassing other features.
Typically, the feature finder systems detect the interfeature pauses, i.e. the blank areas between successive features on the tape on which no signals were recorded, while the tape is advancing at fast forward or rewind speed. The detected interfeature pause is then used to precisely position the selected feature at its beginning at the playback head, and to concomitantly change the tape speed to the playback mode.
However, the feature finder system malfunctions when it mistakes an inter-feature pause for an intra-feature pause, i.e. a silence or very low volume passage occuring during a feature. In order to distinguish between inter-feature and intra-feature pauses, it has been suggested to make the inter-feature pause longer than any intra-feature pause. However, the duration of any pause is a factor of whether the tape is advancing at the playback, or fast forward, or rewind speed, as well as being a factor of where the pause occurs on the tape itself.
To explain in more detail, the playback speed of tape in a capstan-driven tape recorder is constant over the length of the tape However, in the fast forward or rewind mode, the tape is not driven by the capstan, but instead the drive motor is respectively coupled to the takeup reel or supply reel. The tape speed is much faster in the fast forward and rewind modes as compared to the playback mode, thus complicating the detection of the duration of a pause.
For example, assume that a given tape has an inter-feature pause of three seconds duration at playback speed, and also has an intra-feature pause of 0.3 seconds duration. Let us further assume that the fast forward speed is ten times the playback speed. It follows then that the three second inter-feature pause detected during fast forward speed will be picked up in the same period of time as the 0.3 second intra-feature pause detected during playback.
The detection problem is even further compounded by the fact that the tape speed is not constant during either the fast forward or the rewind mode. In fast forward, for example, the takeup reel is driven at a substantially constant rate. The tape speed, however, increases as the advancement of the tape progresses since the diameter of the coil of the tape on the takeup reel increases.
Similarly, in the rewind mode, the supply reel is driven at a substantially constant speed, and as the tape is rewound, the radius of the tape, i.e. the distance of the outermost circumferential portion of the tape from the center of the supply reel, is increased, and concomitantly, the tape speed increases.
Typically, the tape speed at the beginning of a tape, which is advancing at the fast forward or rewind speeds, will be about ten times of the tape speed in the playback mode. The tape speed at the end of the tape depends, of course, on the length of the tape and the size of the reels. For exemplary purposes, the tape speed at the end of a tape can be assumed to be about twenty-five times that of the tape speed in the playback mode.
To repeat the above analysis, an inter-feature pause of three seconds duration will require 3/10=300 milliseconds to pass by the playback head if it is located at the beginning of the tape which is in the fast forward mode. At the end of the tape, the inter-feature pause requires 3/25=120 milliseconds. Hence, the automatic feature finding system must also take into account whether a pause is at the beginning or at the end of a tape.
A standard time period must be established to distinguish between inter-feature and intra-feature pauses. However, if the time period is of fixed time duration, then the automatic feature finding system cannot reliably distinguish between the two types of pauses due to the different tape speeds in different modes of operation, and due to the ever-changing and variable tape speed in the fast forward and rewind modes of operation.
The prior art has proposed various electrical and mechanical feedback circuits for measuring the speed of the reels, detecting the change in the tape speed, and thereupon maintaining the tape speed at a constant value. The prior art has also proposed modifying the tape speed by increasing it at some portions, and slowing it down at some desired portion. However, all of these methods lower the tape running speed. It takes longer to locate the desired feature, and of course, the circuitry for accomplishing the change in the tape speed is very complex.
Another prior art proposal is to update the standard time period each time an inter-feature pause is detected. U.S. Pat. No. 3,723,666 discloses one such system which mechanically switches in different time periods in a pre-set order every time the system detects a pause between features.
However, the known systems have not proven to be altogether satisfactory, because they do not take the tape speed into consideration. For example, by changing the decision period in a pre-set order, the same decision period will serve as the standard no matter whether the tape is started from its beginning, or from the middle of the tape. Furthermore, a certain amount of time is required for a tape to come up to its running speed. If an intra-feature pause is reached before the tape reaches its running speed, then the system will mistake it for an inter-feature pause, and cause the entire system to malfunction.
Another problem associated with the known systems is that the number of inter-feature pauses may not correspond with the number of pre-established time periods. Also, tapes come in different lengths. Hence, the pre-established time periods for a tape having a fifteen minute length is wholly inadequate for another tape having a two hour length. Still another problem is that the system should be able to convert from fast forward mode to rewind mode. However, the known systems having pre-set time periods cannot be easily converted.